not familiar with the fee difference, but i think it might cost less to mail small light weight cards in plastic or pocket cases or small pelican cases or a couple of blu discs in small covers than a harddrive. might be a bit safer or more shock proof. you can add extra layers or protection on cards and not pay that much more than hard drives.
Yeah buy a cheap HD and FedEx- that's the safest, cheapest, best way. Don't ever FedEx your masters and unless you're on a direct Fiber link uploading would take forever.